Process for removing carbon dioxide containing acidic gases from gaseous mixtures using a basic salt activated with a hindered amine

ABSTRACT

Carbon dioxide containing acidic gases are removed from a normally gaseous mixture by a process comprising contacting the normally gaseous mixture with an aqueous solution comprising (a) a basic alkali metal salt or hydroxide and (b) an activator for said basic salt or alkali metal salt or hydroxide comprising at least one sterically hindered amine.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.590,427, filed June 26, 1975, and now abandoned. This application isrelated to U.S. application Ser. No. 750,520, filed Dec. 14, 1976, nowU.S. Pat. No. 4,094,957, issued June 13, 1978; U.S. application Ser. No.768,420, filed Feb. 14, 1977; and U.S. application Ser. No. 768,421,filed Feb. 14, 1977.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved process for removing acidiccomponents from gaseous mixtures containing them and more particularlyrelates to the use of an amine activated basic salt solution forremoving CO₂ containing gases wherein at least one of the amines is asterically hindered amine.

2. Description of the Prior Art

It is well known in the art to treat gases and liquids, such as mixturescontaining acidic gases including CO₂, H₂ S, SO₂, SO₃, CS₂, HCN, COS andoxygen and sulfur derivatives of C₁ to C₄ hydrocarbons with aminesolutions to remove these acidic gases. The amine usually contacts theacidic gases and the liquids as an aqueous solution containing the aminein an absorber tower with the aqueous amine solution contacting theacidic fluid countercurrently.

The acid scrubbing processes known in the art can be generally brokeninto three categories.

The first category is generally referred to as the aqueous amine processwhere relatively large amounts of amine solutions are employed duringthe absorption. This type of process is often utilized in themanufacture of ammonia where nearly complete removal of the acid gas,such as CO₂, is required. It is also used in those instances where anacid gas, such as CO₂, occurs with other acid gases or where the partialpressures of the CO₂ and other gases are low.

A second category is generally referred to as the aqueous base scrubbingprocess or "hot potash" process. In this type of process a small levelof an amine is included as an activator for the aqueous base used in thescrubbing solution. This type of process is generally used where bulkremoval of an acid gas, such as CO₂, is desired. This process alsoapplies to situations where the CO₂ and feed gas pressures are high. Insuch processes, useful results are achieved using aqueous potassiumcarbonate solutions and an amine activator.

A third category is generally referred to as the non-aqueous solventprocess. In this process, water is a minor constituent of the scrubbingsolution and the amine is dissolved in the liquid phase containing thesolvent. In this process, the amine comprises up to 50% of the liquidphase. This type of process is utilized for specialized applicationswhere the partial pressure of CO₂ is extremely high and/or where manyacid gases are present, e.g. COS, CH₃ SH and CS₂.

The present invention pertains to an improved process for practicing thesecond category of acid scrubbing process described above, namely, theaqueous base scrubbing process or "hot potash" process. Many industrialprocesses for removal of acid gases, such as CO₂, use regenerableaqueous alkaline scrubbing solutions, such as an amine and potassiumcarbonate, which are continuously circulated between an absorption zonewhere acid gases are absorbed and a regeneration zone where they aredesorbed usually by steam-stripping. The capital cost of these acidscrubbing processes is generally controlled by the size of theabsorption and regeneration towers, the size of the reboilers forgenerating stripping steam, and the size of the condensers whichcondense spent stripping steam so that condensate may be returned to thesystem to maintain proper water balance. The cost of operating suchscrubbing plants is generally related to the amount of heat required forthe removal of a given amount of acid gas, e.g., thermal efficiency,sometimes expressed as cubic feet of acid gas removed per pound of steamconsumed. Means for reducing the costs in operating these industrialprocesses have focused on the use of absorbing systems or combinationsof chemical absorbents which will operate more efficiently andeffectively in acid gas scrubbing processes using existing equipment.

There are a number of patents which describe improvements to improve theefficiency of the "hot potash" process. Some of these improvements aredescribed below.

In U.S. Pat. No. 2,718,454, there is described a process for usingpotash and similar alkali metal salts in conjunction with amines, suchas monoethanolamine, diethanolamine and triethanolamine to remove acidgases from a gas mixture. The combination of the alkali metal compoundsin conjunction with the designated amine yields higher capacity for acidgases than systems with the amines alone.

In U.S. Pat. No. 3,144,301, there is disclosed the use of potassiumcarbonate in conjunction with diethanolamine and monoethanolamine toremove CO₂ from gaseous mixtures.

In U.S. Pat. Nos. 3,637,345, 3,763,434, and 3,848,057, processes for theremoval of acid gases by means of aqueous carbonate scrubbing solutionsactivated by an amine, such as 1,6-hexanediamine, piperidine and theirderivatives are described.

In U.S Pat. No. 3,856,921, there is disclosed a process for removal ofacid gases from fluids by use of a basic salt of an alkali or alkalineearth metal and an amine activator, such as 2-methylaminoethanol,2-ethylaminoethanol, morpholine, pyrrolidine and derivatives thereof.

U.S. Pat. Nos. 3,563,695, 3,563,696, and 3,642,430 to Benson discloseprocesses for removing CO₂ and H₂ S from gaseous mixtures by alkalinescrubbing processes wherein at least two separate regeneration zones areprovided. Alkanolamines and aminoacids are described as activators, butthe use of sterically hindered amines is not taught or disclosed inthese patents.

Belgian Pat. No. 767,105 discloses a process for removing acid gasesfrom gaseous streams by contacting the gaseous streams with a solutioncomprising potassium carbonate and an aminoacid, such as substitutedglycines (e.g., N-isopropyl glycine, N-t-butylglycine,N-cyclohexylglycine, etc.). The data in Table IV of the patent indicatesthat the highly substituted compounds, such as N-t-butylglycine, areinferior to the straight chain compounds, such as N-n-butylglycine butN-cyclohexyl glycine, a sterically hindered amine, has a good rate ofabsorption.

British Pat. No. 1,063,517 to Allen G. Eickmeyer discloses removal ofacid gases, such as CO₂ and H₂ S, from gaseous mixtures by the use ofpotassium carbonate and other alkali metal carbonates in conjunctionwith particular amines which will avoid corrosion problems and at thesame time accelerate the absorption and subsequent desorption of the CO₂and H₂ S. Specifically disclosed amines are ethylene polyamine,alkanolamines, or alkanolamine borates as well as mixtures thereof.Examples of such amines are ethylenediamine, diethylenetriamine anddiethanolamine.

British Pat. No. 1,218,083 describes a process for removing acid gasesfrom feed streams by contacting the feed streams with a compositioncomprising an alkaline salt, such as potassium carbonate, and analkanolamine such as diisopropanolamine.

British Pat. No. 1,238,696 discloses a process for removing acid gasesfrom feed streams by contacting the feed streams with a compositioncomprising an organic solvent and an alkanolamine, such ascyclohexylaminoethanol. The patent does not disclose the advantages ofusing sterically hindered amines to improve working capacity.

British Pat. No. 1,305,718 describes a process for removing acid gasesfrom gaseous streams by contacting the same with an absorbing solutioncomprising a solution of an alkaline salt of an alkali metal andregeneration of the absorbing solution wherein the absorbing solutionincludes a minor amount of a substituted or unsubstituted aminoacid.Examples of the aminoacids described are N-ethyl-3-amino propionic acid,N-ethyl-4-amino butyric acid, and N-ethyl-6-amino hexanoic acid.

U.S. Pat. No. 2,176,441 to Ulrich et al. teaches the use of aminoacidshaving a primary, secondary or tertiary amino group and at least twonitrogen atoms to remove acidic gases. The patentees provide variousgeneral formulae for the aminoacids taught to be useful in the acid gasscrubbing process. While certain "sterically hindered amines" can bederived by proper choice of substituent groups in the general formulaethere is no teaching that the sterically hindered amines will achieveany unexpected results, such as improved working capacity.

U.S. Pat. No. 1,783,901 to Bottoms teaches the use of aliphatic aminecompounds including alkanolamines such as triethanolamine in an aqueousamine scrubbing process. The patent does not mention the use ofsterically hindered amines.

U.S Pat. Nos. 2,139,122, 2,139,123 and 2,139,124 to Haas et al. discloseaminoalcohols (U.S. Pat. No. 2,139,122 discloses2-amino-2-methyl-1-propanol) and in page 2, column 2, it is disclosedthat these aminoalcohols, due to their basic nature may be utilized toabsorb acids such as hydrogen sulfide or carbon dioxide from industrialgases.

Canadian Pat. No. 619,193 teaches the use of various aqueous solutionscontaining specific amino compounds for the removal of acidic gases,such as CO₂, from gaseous feed streams. Careful choice of the various R₁and R₂ groups in the formula in column 4, lines 35-40, will revealsterically hindered amines. However, there is no teaching that theseamines give improved results or working capacity compared tononsterically hindered amines.

Prior art workers have taught that sterically hindered amines would havelow rates of combination with CO₂ and apparently concluded, althoughother explanations are possible, that such sterically hindered amineswould be inefficient in CO₂ scrubbing processes. For example, Sharma, M.M., Trans. Faraday Soc., 61, 681-8 (1965) described the kinetics ofreaction of CO₂ and COS with 38 amines, some of which are stericallyhindered amines. Other researchers have attributed relatively poorabsorption rates of CO₂ by amines to steric hindrance. See, for example,J. L. Frahn and J. A. Mills, Aust. J. Chem., 17, 256-73 (1964) and M. B.Jensen, Acta Chemica Scandinavica, 11, 499-505 (1957).

Shrier and Danckwerts, Ind. Eng. Chem. Fundamentals, 8, 415 (1969)discussed the use of amines as promoters for aqueous carbon dioxideabsorption solutions. However, these researchers only ran initialabsorption experiments and did not recognize the unique capacityadvantages obtained by using sterically hindered amines in an acid gasscrubbing process. Also of interest is Danckwerts and Sharma, TheChemical Engineer, October 1966, pp. 244-280.

In the prior art discussed above, it is apparent that the efficiency ofprocesses employing absorbing solutions is generally limited by therelatively slow rate of transfer of molecules of the acid gas from thegas phase to the liquid phase as well as in the regeneration of theabsorbing solution. Many of the above-described prior art processes dealwith means to render the acid gas scrubbing process more efficient.

It has now been discovered that sterically hindered amines unexpectedlyimprove the efficiency, effectiveness and cyclic working capacity of theacid gas scrubbing processes in all three of the above-mentioned processcategories. In the case of the sterically hindered amine activated "hotpotash" CO₂ containing acid gas scrubbing process of the presentinvention the process can be operated at a cyclic working capacitysignificantly greater than when diethanolamine or 1,6-hexanediamine isthe amine activator used in a similar process. As explained in greaterdetail hereafter, it is postulated that the increase in cyclic capacityobserved with the sterically hindered amines is due to the instabilityof their carbamates. In that respect, sterically hindered amines aresimilar to tertiary amines. Tertiary amines are not used on a commercialscale for carbon dioxide containing acid gas scrubbing due to their lowrates of absorption and desorption.

SUMMARY OF THE INVENTION

In one embodiment of the present invention there is provided a processfor the removal of CO₂ from gaseous feeds containing CO₂, whichcomprises contacting said gaseous feeds with an aqueous scrubbingsolution comprising (a) an alkaline material comprising a basic alkalimetal salt or alkali metal hydroxide and (b) an activator for said basicsalt or hydroxide comprising at least one sterically hindered amine.

In another embodiment of the invention there is provided a process forthe removal of CO₂ from gaseous feeds containing CO₂ which comprises, insequential steps:

(1) contacting said feed with an aqueous scrubbing solution comprising(a) an alkaline material comprising a basic alkali metal salt or alkalimetal hydroxide in an amount ranging from about 10 to about 40 wt. %based on the weight of said scrubbing solution, and (b) at least 2 wt. %of an amine activator, at conditions whereby said acidic components areabsorbed in said scrubbing solution, and

(2) regenerating said scrubbing solution at conditions whereby said CO₂is desorbed from said scrubbing solution, the improvement whichcomprises:

providing at least one sterically hindered amine in said scrubbingsolution as said amine activator, and

operating said process at conditions whereby the difference between themoles of CO₂ absorbed at the end of step (1) (absorption step) and themoles of CO₂ absorbed at the end of step (2)(desorption step) would begreater, preferably at least about 15% greater, and more preferably 20to 60% greater, at the thermodynamic equilibrium (as determined from thevapor-liquid equilibrium isotherm in the reaction mixture) than in anaqueous scrubbing process wherein diethanolamine or 1,6-hexanediamine isthe only amine activator utilized under similar conditions of gaseousfeed composition, scrubbing solution composition, temperatures andpressures.

Various aqueous solutions which are known in the art for the absorptionof carbon dioxide from gaseous mixtures may be found in U.S. Pat. No.3,793,434, herein incorporated by reference.

In general, the aqueous scrubbing solution will comprise an alkalinematerial comprising a basic alkali metal salt or alkali metal hydroxideselected from the Group IA of the Periodic Table of Elements. Morepreferably, the aqueous scrubbing solution comprises potassium or sodiumborate, carbonate, hydroxide, phosphate or bicarbonate. Most preferably,the aqueous solution comprises potassium carbonate.

The alkaline material comprising a basic alkali metal salt or alkalimetal hydroxide may be present in the scrubbing solution at a weightpercent of from 10 to 40, more preferably from 20 to 35 weight percent.In aqueous scrubbing processes the concentration of the alkalinematerial will be selected so as to remain in solution throughout theentire cycle of absorption of CO₂ from the gas stream and desorption ofCO₂ in the regeneration step.

The sterically hindered amine activators used in the practice of thepresent invention are compounds containing at least one secondary aminogroup attached to either a secondary or tertiary carbon atom or aprimary amino group attached to a tertiary carbon atom. These amineactivators are selected to be at least partly soluble in the particularsolvent used. In aqueous solutions the sterically hindered amineactivator will preferably additionally comprise one or morewater-solubilizing groups which may be selected from the groupconsisting of amino groups, i.e. additional sterically hindered aminogroups, hydroxy groups and carboxyl groups. At least one nitrogen atomwill have a sterically hindered structure as described above. Asdescribed in more detail hereinafter, the sterically hindered amineactivators are most preferably selected from the group consisting ofaminoethers, aminoalcohols, di- and triamines.

In still another embodiment of the present invention there is providedan aqueous scrubbing solution comprising an aqueous mixture containing10 to about 40 weight percent of an alkaline material comprising a basicalkali metal salt or alkali metal hydroxide selected from Group IA ofthe Periodic Table of the Elements, preferably potassium or sodiumborate, carbonate, hydroxide, phosphate or bicarbonate, and mostpreferably potassium carbonate, and 2 to about 20 weight percent of atleast one sterically hindered amine activator, wherein the stericallyhindered amine is defined as being a compound having at least onesecondary amino group attached to either a secondary or tertiary carbonatom or a primary amino group attached to a tertiary carbon atom. Thesterically hindered amine activator is preferably selected from thegroup consisting of aminoethers, aminoalcohols, di- and triamines, andmost preferably selected from the group consisting ofN-cyclohexyl1,3-propanediamine, 1,8-p-menthanediamine, 1,7-bis-sec.butyl-diethylenetriamine, 2,2,5,5-tetramethyl diethylenetriamine,3-amino-3-methyl-1-butanol, 2-amino-2-methyl-1-propanol, N₁-isopropyl-2-methyl-propanediamine; N₁(1,1-dimethyl-2-hydroxyethyl)-2-methyl-1,2-propanediamine; and N₁-isopropyl-N₂ -(3-aminopropyl)-2-methyl-1,2-propanediamine. Especiallypreferred is N-cyclohexyl-1,3-propanediamine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic flow sheet illustrating an experimentalreaction apparatus for removing carbon dioxide from gaseous streams.

FIG. 2 graphically illustrates the vapor-liquid equilibrium isothermsfor potassium carbonate solutions activated by equal total nitrogencontents of diethanolamine (prior art amine activator) and1,8-p-menthane diamine (a sterically hindered amine activator of theinvention) at 250° F. (121.1° C.) wherein the CO₂ partial pressure is afunction of the carbonate conversion.

FIG. 3 graphically illustrates the vapor-liquid equilibrium isothermsfor amine-promoted K₂ CO₃ scrubbing solutions illustrating thecalculation of thermodynamic cyclic capacity of systems shown in FIG. 2,i.e., potassium carbonate solutions activated by equal nitrogen contentsof diethanolamine (prior art amine activator) and 1,8-p-menthane-diamine(a sterically hindered amine activator of the invention) at 250° F.(121.1° C.) wherein the CO₂ partial pressure is a function of thecarbonate conversion.

FIG. 4 graphically illustrates the vapor-liquid equilibrium isothermsfor potassium carbonate solutions activated by equal total nitrogencontents of diethanolamine (prior art amine activator) andN-cyclohexyl-1,3-propanediamine (a sterically hindered amine activatorof the invention) at 250° F. (121.1° C.) wherein the CO₂ partialpressure is a function of the carbonate conversion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The term carbon dioxide containing acidic gas feeds also includes H₂ S,SO₂, SO₃, CS₂, HCN, COS and the oxygen and sulfur derivatives of C₁ toC₄ hydrocarbons in various amounts as they frequently appear in gaseousmixtures. These acid gases other than the carbon dioxide may be presentin trace amounts within a gaseous mixture or feed.

The contacting of the absorbent mixture and the feed gas may take placein any suitable contacting tower. In such processes, the gaseous mixturefrom which the CO₂ is to be removed may be brought into intimate contactwith the absorbing solution using conventional means, such as a towerpacked with, for example, ceramic rings or with bubble cap plates orsieve plates, or a bubble reactor.

In a preferred mode of practicing the invention, the absorption step isconducted by feeding the gaseous mixture into the base of the towerwhile fresh absorbing solution is fed into the top. The gaseous mixturefreed largely from acid gases e.g., CO₂ emerges from the top.Preferably, the temperature of the absorbing solution during theabsorption step is in the range from about 25° to about 200° C., andmore preferably from 35° to about 150° C. Pressures may vary widely;acceptable pressures are between 5 and 2000 psig, preferably 100 to 1500psig, and most preferably 200 to 1000 psig in the absorber. In thedesorber the pressures will range from about 5 to 100 psig. The partialpressure of the acid gas, e.g., CO₂ in the feed mixture will preferablybe in the range from about 0.1 to about 500 psia, and more preferably inthe range from about 1 to about 400 psia. The contacting takes placeunder conditions such that the acid gas, e.g., CO₂, is absorbed by thesolution. Generally, the countercurrent contacting to remove the acidgases, e.g., CO₂ will last for a period of from 0.1 to 60 minutes,preferably 1 to 5 minutes. During absorption, the solution is preferablymaintained in a single phase.

The absorbing solution comprising the aqueous mixture containing thebasic alkali metal salt and sterically hindered amine which is saturatedor partially saturated with gases, such as CO₂ and H₂ S may beregenerated so that it may be recycled back to the absorber. Theregeneration should also preferably take place in a single liquid phase.The regeneration or desorption is accomplished by conventional means,such as pressure reduction, which causes the acid gases e.g., CO₂ and H₂S to flash off or by passing the solution into a tower of similarconstruction to that used in the absorption step, at or near the top ofthe tower, and passing an inert gas such as air or nitrogen orpreferably steam up the tower. The temperature of the solution duringthe regeneration step may be the same as used in the absorbing step,i.e. 25° to about 200° C. and preferably 35° to about 150° C. Theabsorbing solution, after being cleansed of at least a portion of theacid bodies, may be recycled back to the absorbing tower. Makeupabsorbent may be added as needed.

For example, during desorption, the acid gas, e.g., CO₂ -rich solutionfrom the high pressure absorber is sent first to a flash chamber wheresteam and some CO₂ are flashed from solution at low pressure. The amountof CO₂ flashed off will in general be about 35 to 40% of the net CO₂recovered in the flash and stripper. Solution from the flash drum isthen stream stripped in the packed or plate tower, stripping steamhaving been generated in the reboiler in the base of the stripper.Pressure in the flash drum and stripper is usually 16 to about 100 psia,preferably 16 to about 30 psia, and the temperature is in the range fromabout 25° to about 200° C., preferably 35° to about 150° C., and morepreferably 100° to about 140° C. Stripper and flash temperatures will,of course, depend on stripper pressure, thus at about 16 to 25 psiastripper pressures, the temperature will preferably be about 100° toabout 140° C. during desorption. The solution is preferably maintainedas a single phase during desorption.

In the most preferred embodiment of the present invention, the acidgases, e.g., carbon dioxide containing gases, are removed from a gaseousfeed containing carbon dioxide by means of a process which comprises, insequential steps:

(1) contacting the gaseous feed with an aqueous scrubbing solutioncomprising (a) potassium carbonate in an amount ranging from about 10 toabout 40 weight percent, preferably 20 to about 35 weight percent, basedon the weight of the scrubbing solution, (b) an amine activator, in anamount ranging from about 2 to about 20 weight percent, preferably 5 toabout 15 weight percent, based on the weight of the scrubbing solution,said amine activator being comprised of at least one sterically hinderedamine, said sterically hindered amine being defined as containing atleast one secondary amine group attached to either a secondary ortertiary carbon atom or a primary amino group attached to a tertiarycarbon atom and being selected from the group consisting of aminoethers,aminoalcohols, di- and triamines, the balance of said scrubbing solutionbeing comprised of water, said contacting being conducted at conditionswhereby the acid gases e.g., CO₂ are absorbed in said scrubbingsolution, and preferably at a temperature ranging from 25° to about 200°C., more preferably ranging from about 35° to about 150° C., and apressure ranging from 100 to about 1500 psig, and

(2) regenerating said solution at conditions whereby said acid gases aredesorbed from said scrubbing solution, and preferably at a temperatureranging from 25° to about 200° C., more preferably ranging from about35° to about 150° C., and a pressure from 16 to about 100 psia.

The time of contacting the gaseous mixture with the amine-promotedscrubbing solution of the invention may vary from a few seconds tohours, for example, 15 minutes.

After contacting the gaseous mixture with the amine-promoted scrubbingsolution until the capacity of at least about 80% or preferably at leastabout 90% of the solution's capacity is depleted, it must beregenerated. The capacity of the amine-promoted scrubbing solutionincludes the capacity of the dissolved alkali metal salt, e.g.,potassium carbonate for reacting with the carbon dioxide as well as thecapacity of the sterically hindered amine of the invention for thecarbon dioxide. Regeneration of the amine-promoted scrubbing solutionmay be accomplished by decreasing the pressure and/or increasing thetemperature of the amine-promoted scrubbing solution to a point at whichthe absorbed carbon dioxide flashes off. The addition of an inert gas,e.g., N₂ or steam during the regeneration of the amine-promotedscrubbing solution is also within the scope of the present invention.The process of regeneration is more particularly described in U.S. Pat.No. 3,848,057, herein incorporated by reference. The sterically hinderedamines of the present invention allow a more complete desorption ascompared to the prior art amine-promoted scrubbing solutions, e.g.,diethanolamine or 1,6-hexanediamine promoted potassium carbonate rununder the same desorption conditions. Thus, savings in the steamutilized to heat and purge the amine-promoted scrubbing solution duringregeneration are obtained.

DETAILED DESCRIPTION OF THE SCRUBBING SOLUTION

It has been known in the past to use alkali metal carbonates,bicarbonates, hydroxides, borates and phosphates for the absorption ofthe CO₂ containing acidic gases from a fluid or gaseous mixture. Thesesalts may be used in the present invention. Typical salts which may beutilized in the practice of the present invention include sodiumhydroxide, sodium carbonate, lithium carbonate, potassium hydroxide,potassium bicarbonate, potassium carbonate and cesium carbonate,potassium carbonate being most preferred.

The sterically hindered amines used as activators in the scrubbingsolution of the present invention are preferably defined as those aminescontaining at least one secondary amino group attached to either asecondary or tertiary carbon atom or a primary amino group attached to atertiary carbon atom. The sterically hindered amine activators of theinvention are preferably selected from the group consisting ofaminoethers, aminoalcohols, di- and triamines.

The sterically hindered amine of the instant invention will preferablybe selected according to other parameters besides steric hindrance.preferably, the sterically hindered amines will be asymmetricalcompounds inasmuch as the symmetrical amines have a tendency to givesolid precipitates in the aqueous alkaline compositions utilized in thetype of process involved in the instant invention. Such has been thecase observed with N,N'-di-isopropyl-ethylenediamine and2,5-diamino-2,5-dimethylhexane. The precipitate could be a bicarbonateor bicarbamate, easily undergoing crystallization owing to thesymmetrical structure of these compounds. The precipitate was notobserved in the case of asymmetric sterically hindered amines, such as1,8-p-menthanediamine and 2,2,5,5-tetramethyldiethylenetriamine. In thecase of 1,7-di-sec.-butyl-diethylenetriamine, precipitation was notobserved, although the molecule is symmetric. However, the degree ofsymmetry for this compound is lower than in the case ofN,N'-diisopropyl-ethylenediamine and 2,5-diamino-2,5-dimethylhexane.Besides, the central amino group, being sterically unhindered, easilyundergoes carbamation, thereby increasing solubility and hinderingprecipitation.

It is also important in choosing an amine for use in the process of theinstant invention that it have low volatility so that it will not belost during absorption and desorption. The sterically hindered aminegenerally should have a boiling point of at least 100° C., preferably atleast 180° C. The amine used need not be completely soluble in watersince partial immiscibility can increase the rate of carbon dioxideabsorption from the gaseous mixture as it increases the total contactsurface between liquid and gas. On the other hand, if the amine isinsoluble in the water phase over most of the carbonation ratiointerval, that can create a problem in an industrial absorber. Such isthe case with 1,8-p-menthanediamine, although the appropriate selectionof an emulsifier is believed to solve this problem. The presence ofsterically unhindered amino groups can be useful as they easily undergocarbamation, thereby reducing volatility. The melting point should belower than 60°-70° C. Of course, the amine should be stable under thereaction conditions.

Monoamines are not generally suitable as activators in the process ofthe instant invention because the lower members of the family are toovolatile and the higher members have limited solubility.Monoaminoalcohols and monoaminoacids are much less volatile and muchmore soluble than the corresponding monoamines containing the samenumber of carbon atoms. However, it has been observed that somemonoaminoalcohols and monoaminoacids lead to lower rate increases thanthose observed with the diamines, presumably owing to hydrogen-bondstabilization of the carbamates.

Nonlimiting examples of the sterically hindered amines useful aspromoters for the scrubbing solution of the present invention include:

    ______________________________________                                        Diamines                                                                      Monoprimary aliphatic diamines                                                 ##STR1##                                                                     N.sub.1 -tert. butyl-1,4-pentanediamine                                        ##STR2##                                                                     N.sub.2 -isopropyl-4-methyl-2,4-pentanediamine                                 ##STR3##                                                                     N.sub.1 -isopropyl-2-methyl-1,2-butanediamine                                  ##STR4##                                                                     2-ethylamino-2-methyl-4-aminopentane                                           ##STR5##                                                                     N-tert. pentyl-1,4-butanediamine                                               ##STR6##                                                                     N-tert. butyl-1,5-pentanediamine                                               ##STR7##                                                                     N.sub.2 -isopropyl-2-methyl-1,2-propanediamine                                 ##STR8##                                                                     N-sec. butyl-1,3-propanediamine                                                ##STR9##                                                                     N.sub.1 -dimethyl-1,2-diamino-2-methylbutane                                   ##STR10##                                                                    N-t-butyl-ethylenediamine                                                      ##STR11##                                                                    N-t-butyl-1,3-propanediamine                                                   ##STR12##                                                                    2-methylamino-2-methyl-4-amino pentane                                         ##STR13##                                                                    N.sub.1 -t-butyl-2-methyl-1,2-propanediamine                                   ##STR14##                                                                    N.sub.1 -butyl-2-methyl-1,2-propanediamine                                     ##STR15##                                                                    N-sec. butyl-2-methyl-1,3-propanediamine                                       ##STR16##                                                                    N.sub.1 -propyl-2-methyl-1,2-propanediamine                                    ##STR17##                                                                    N.sub.1 -sec. butyl-2-methyl-1,2-propanediamine                                ##STR18##                                                                    N-t-butyl-1,4-butanediamine                                                    ##STR19##                                                                    N.sub.2 -ethyl-1,2-hexanediamine                                              Arylaliphatic diamines in which the amino groups are                          separated by up to 5 or more than 6 carbon atoms:                              ##STR20##                                                                    1-methyl-1-phenyl ethylenediamine                                              ##STR21##                                                                    2-benzyl-1,2-propanediamine                                                    ##STR22##                                                                    1-phenyl-1(2-amino-ethylamino)-propane                                         ##STR23##                                                                    N.sub.1 -methyl-2-phenyl-1,2-butanediamine                                    Cycloaliphatic diamines:                                                       ##STR24##                                                                    N.sub.1 -cyclohexyl-1,2-propanediamine                                         ##STR25##                                                                    1-amino-1-(2-amino-isopropyl)-cyclohexane                                      ##STR26##                                                                    1-methylamino-1-aminomethyl-cyclopentane                                       ##STR27##                                                                    1-amino-1-aminomethylcycloheptane                                              ##STR28##                                                                    N-isopropyl-1,2-diaminocyclohexane                                             ##STR29##                                                                    N.sub.2 -cyclohexyl-1,2-butanediamine                                          ##STR30##                                                                    N.sub.2 -cyclohexyl-1,2-propanediamine                                         ##STR31##                                                                    N-cycloheptyl-ethylenediamine                                                  ##STR32##                                                                    N.sub.1 -cyclohexyl-2-methyl-1,2-propanediamine                                ##STR33##                                                                    1-(2-aminoisopropyl)-2-amino-3-methyl-cyclopentane                             ##STR34##                                                                    N-isopropyl-1,4-diaminocyclohexane                                             ##STR35##                                                                    N.sub.1 -cyclohexyl-N.sub.2 -methyl-ethylenediamine                            ##STR36##                                                                    N-cyclohexyl-ethylenediamine                                                   ##STR37##                                                                    N.sub.1 -cyclohexyl-N.sub.2 -ethyl-ethylenediamine                             ##STR38##                                                                    N.sub.1 -cyclohexyl-N.sub.2 -methyl-1,2-propanediamine                         ##STR39##                                                                    N-cyclohexyl-1,3-propanediamine                                                ##STR40##                                                                    1,8-p-menthanediamine                                                          ##STR41##                                                                    1-amino-1-aminomethylcyclohexane                                               ##STR42##                                                                    1,3-diamino-1-methylcyclohexane                                                ##STR43##                                                                    N.sub.2 -cyclohexyl-2-methyl-1,2-propanediamine                               Biprimary aliphatic diamines in which the nitrogen atoms                      are separated by up to 5 or more than 6 carbon atoms:                          ##STR44##                                                                    2,4-diamino-2-methylpentane                                                    ##STR45##                                                                    3,5-diamino-3-methylheptane                                                   Disecondary aliphatic diamines:                                                ##STR46##                                                                    N.sub.1 -tert. butyl-N.sub.2 -isopropyl-1,3-propanediamine                     ##STR47##                                                                    N.sub.1 -tert. butyl-N.sub.2 -sec. butyl-ethylenediamine                       ##STR48##                                                                    N.sub.1 -tert. butyl-N.sub.2 -isopropyl-1,3-propanediamine                     ##STR49##                                                                    N.sub.1 -tert. butyl-N.sub.2 -butyl-ethylenediamine                            ##STR50##                                                                    N.sub.1 -tert. butyl-N.sub.2 -isobutyl-ethylenediamine                         ##STR51##                                                                    N.sub.1 N.sub.2 -diisopropyl-1,2-propanediamine                                ##STR52##                                                                    N.sub.1 tert. butyl-N.sub.2 -isopropyl-ethylenediamine                         ##STR53##                                                                    N.sub.1 -sec. butyl-N.sub.2 -isopropyl-ethylenediamine                         ##STR54##                                                                    N.sub.1 -tert. pentyl-N.sub.2 -isopropyl-ethylenediamine                       ##STR55##                                                                    N.sub.1,N.sub.3 -diethyl-1,3-butanediamine                                     ##STR56##                                                                    N.sub.1 -tert. butyl-N.sub.1 -methyl-ethylenediamine                           ##STR57##                                                                    N.sub.1 -(2-pentyl)-N.sub.2 -methyl-ethylenediamine                            ##STR58##                                                                    N.sub.1 -tert. butyl-N.sub.2 -methyl-1,4-butanediamine                         ##STR59##                                                                    N.sub.1 -tert. butyl-N.sub.2 -ethyl-1,3-propanediamine                        Secondary-tertiary diamines:                                                   ##STR60##                                                                    N.sub.1 -tert. butyl-N.sub.2 -diethyl-ethylenediamine                         Triamines                                                                     Cycloaliphatic triamines:                                                      ##STR61##                                                                    N-cyclohexyl-dipropylene triamine                                             Aliphatic triamines containing at most one primary amino                      ______________________________________                                        group:                                                                         ##STR62##                                                                    N.sub.1,N.sub.3,2-pentamethyl-1,2,3-triaminopropane                            ##STR63##                                                                    N.sub.1 -isopropyl-N.sub.2 -(3-aminopropyl)-2-methyl-1,2-propanediamine       Biprimary or triprimary aliphatic triamines, in                               which any two vicinal amino groups are separated                              by up to 5 or more than 6 carbon atoms:                                        ##STR64##                                                                    2,2-dimethyl-diethylenetriamine                                                ##STR65##                                                                    N.sub.1 -tert. butyl-1,2,3-triaminopropane                                     ##STR66##                                                                    2,2,5,5-tetramethyldiethylenetriamine                                         Aminoalcohols                                                                 Cycloaliphatic diaminoalcohols:                                                ##STR67##                                                                    1-amino-1-aminomethyl-2-hydroxymethyl-cyclohexane                              ##STR68##                                                                    N-hydroxyethyl-1,2-diaminocyclohexane                                          ##STR69##                                                                    N-cyclohexyl-1,3-diamino-2-propanol                                            ##STR70##                                                                    N-(2-hydroxycyclohexyl)-1,3-propanediamine                                     ##STR71##                                                                    N-isopropanol-1,2-diaminocyclohexane                                           ##STR72##                                                                    N-(2-hydroxybutyl)-1,4-diaminocyclohexane                                     Diaminoalcohols containing at most one primary amino group:                   ______________________________________                                         ##STR73##                                                                    N.sub.1 (1-hydroxy-2-butyl)-2-methyl-1,2-propanediamine                        ##STR74##                                                                    N(1-hydroxy-2-methyl-2-butyl)-1,3-propanediamine                               ##STR75##                                                                    N.sub.1 (1,1-dimethyl-2-hydroxyethyl)-2-methyl-1,2-propanediamine              ##STR76##                                                                    N.sub.3 -isobutyl-2-methyl-2,3-diamino-1-propanol                              ##STR77##                                                                    N(3-hydroxy-2-butyl)-2,3-diaminobutane                                         ##STR78##                                                                    N.sub.1 -hydroxyethyl-2-methyl-1,2-propanediamine                              ##STR79##                                                                    2,N.sub.3,N.sub.3 -trimethyl-2,3-diamino-1-propanol                            ##STR80##                                                                    N.sub.1,2-dimethyl-N.sub.1 -hydroxyethyl-1,2-propanediamine                    ##STR81##                                                                    N(1,1-dimethyl-2-hydroxyethyl)-1,3-propanediamine                              ##STR82##                                                                    N-tert. butyl-1,3-diamino-2-propanol                                          Sterically hindered primary monoaminoalcohols                                  ##STR83##                                                                    3-amino-3-methyl-2-pentanol                                                    ##STR84##                                                                    1-hydroxymethyl-cyclopentylamine                                               ##STR85##                                                                    2,3-dimethyl-3-amino-1-butanol                                                 ##STR86##                                                                    2-amino-2-ethyl-1-butanol                                                      ##STR87##                                                                    1-methyl-2-hydroxycyclopentylamine                                             ##STR88##                                                                    2-amino-2-methyl-3-pentanol                                                    ##STR89##                                                                    2,4-dimethyl-2-amino cyclohexanol                                              ##STR90##                                                                    1-hydroxyethyl cyclohexylamine                                                 ##STR91##                                                                    1-hydroxymethyl-3-methyl cyclohexylamine                                       ##STR92##                                                                    2-hydroxymethyl-1-methyl cyclohexylamine                                       ##STR93##                                                                    2-amino-2-methyl-1-propanol                                                    ##STR94##                                                                    2-amino-2-methyl-1-butanol                                                     ##STR95##                                                                    3-amino-3-methyl-1-butanol                                                     ##STR96##                                                                    3-amino-3-methyl-2-butanol                                                     ##STR97##                                                                    2-amino-2,3-dimethyl-3-butanol                                                 ##STR98##                                                                    2-amino-2,3-dimethyl-1-butanol                                                 ##STR99##                                                                    1-hydroxymethyl-cyclohexylamine                                               Amino ethers                                                                  Sterically hindered amino-hydroxyalkyl ethers                                  ##STR100##                                                                   2(2-amino-2-methylpropoxy)-ethanol                                            2-substituted piperidines                                                      ##STR101##                                                                   2-piperidine methanol                                                          ##STR102##                                                                   2-piperidine ethanol                                                           ##STR103##                                                                   2-(1-hydroxyethyl)-piperidine                                                  ##STR104##                                                                   5-hydroxy-2-methyl piperidine                                                  ##STR105##                                                                   2-methyl-3-hydroxy piperidine                                                  ##STR106##                                                                   2,6-dimethyl-3-hydroxy piperidine                                              ##STR107##                                                                   2,5-dimethyl-4-hydroxy piperidine                                             Piperazine derivatives:                                                        ##STR108##                                                                   2-tert. butylamino-methyl-1,4-dimethylpiperazine                              Azacyclo alkanes                                                               ##STR109##                                                                   1-tert. butylamino-3,6-dimethyl-3,6-diaza-cycloheptane                        Aminoacids                                                                     ##STR110##                                                                   N-cyclohexyl-beta-alanine                                                     ______________________________________                                    

The most preferred sterically hindered amines to be used as activatorsfor the alkaline absorbents in the practice of the present inventioninclude: N-cyclohexyl-1,3-propanediamine; 1,8-p-menthanediamine;1,7-bis-secbutyl-diethylenetriamine; 2,2,5,5-tetramethyldiethylenetriamine; 3-amino-3-methyl-1-butanol;2-amino-2-methyl-1-propanol; N₁ -isopropyl-2-methyl-1,2-propanediamine;N₁ -(1,1-dimethyl-2-hydroxyethyl)-2-methyl-1,2-propanediamine; and N₁-isopropyl-N₂ -(3-aminopropyl)-2-methyl-1,2-propanediamine.

It should be noted that, as used throughout the instant specification, ascrubbing solution which does not comprise the sterically hinderedamines promoters as defined above, may comprise a non-stericallyhindered amine, e.g., 1,6-hexanediamine, mono- and diethanolamine, etc.A comparison of scrubbing processes utilizing, for example, aqueous K₂CO₃ promoted with amines, will show that when a sterically hinderedamine is used, a working capacity increase of 20% or greater may beobtained over a non-sterically hindered amine.

The aqueous scrubbing solution used in the practice of the presentinvention will be comprised of a major proportion of an alkalinematerial comprising alkali metal salts or alkali metal hydroxides and aminor proportion of the amine activator comprising the stericallyhindered amine. The remainder of the solution will be comprised of waterand other commonly used additives such as antifoaming agents,antioxidants, corrosion inhibitors, etc. the incorporation of which iswithin the skill of the artisan. For example, the aqueous scrubbingsolutions of the instant invention may comprise arsenious anhydride,selenious and tellurous acid, protides, amino acids, e.g., glycine,vanadium oxides, e.g., V₂ O₃, chromates, e.g., K₂ Cr₂ O₇, etc.

By practicing the process of the present invention one can operate theprocess as described above at conditions whereby the thermodynamiccyclic capacity (also referred to as "working capacity") (as determinedfrom the vapor-liquid equilibrium isotherm in the reaction mixture)would be greater, preferably at least 15% greater and more preferably 20to 60% greater than in an aqueous scrubbing amine activated or promotedpotassium carbonate process wherein diethanolamine or 1,6-hexanediamineis the only amine activator utilized under similar conditions of gaseousfeed composition, scrubbing solution composition, temperatures andpressures. The thermodynamic cyclic capacity or working capacity for agiven CO₂ gas scrubbing process is the difference in the moles of CO₂gas absorbed in the solution at the termination of the absorption anddesorption steps, based on the moles of the basic alkali metal salt orhydroxide, e.g., K₂ CO₃ originally present. In other words, workingcapacity is defined as follows: ##EQU1##

It should be noted that throughout the specification wherein the terms"working capacity" or "cyclic capacity" are referred to, these terms maybe defined as the difference between CO₂ loading in solution atabsorption conditions (step 1) and the CO₂ loading in solution atregeneration conditions (step 2), each divided by the initial moles ofK₂ CO₃ in the scrubbing solution. The term "working capacity" is to beconstrued as synonymous and relates to the "thermodynamic cycliccapacity", that is the loading is measured at equilibrium conditions.This working capacity may be obtained from the vapor-liquid equilibriumisotherm, that is from the relation between the CO₂ pressure in the gasand the acid gas, e.g., CO₂ loading in the solution at equilibrium at agiven temperature. To calculate thermodynamic cyclic capacity, thefollowing parameters must usually be specified: (1) acid gas, e.g., CO₂absorption pressure, (2) acid gas, e.g., CO₂ regeneration pressure, (3)temperature of absorption, (4) temperature of regeneration, (5)scrubbing solution composition, that is, weight percent amine and theweight percent of the alkaline salt of hydroxide, for example, potassiumcarbonate, and (6) gas composition. The use of these parameters todescribe the improved process of the instant invention is documented inthe examples below, and in FIGS. 2, 3 and 4. However, the skilledartisan may conveniently demonstrate the improved process which resultsby use of a sterically hindered amine by a comparison directly with aprocess wherein the sterically hindered amine is not included as theamine activator or promoter in the aqueous scrubbing solution. Forexample, it will be found when comparing two similar CO₂ scrubbingprocesses (that is similar gas composition, similar scrubbing solutioncomposition, similar pressure and temperature conditions) that when thesterically hindered amines of the instant invention are utilized asactivators or promoters the difference between the amount of CO₂absorbed at the end of step 1 (absorption step) defined above and step 2(desorption step) defined above is significantly greater than in anamine activated "hot pot" process wherein the amine activator isdiethanolamine or 1,6-hexanediamine. This significantly increasedworking capacity is observed even when the scrubbing solution that isbeing compared comprises an equimolar amount of another prior art aminepromoter such as diethanolamine or 1,6-hexanediamine. It has been foundthat the use of the sterically hindered amines of the instant inventiongives a thermodynamic cyclic capacity (working capacity) which is atleast 15% greater than the working capacity of a scrubbing solutionwhich does not utilize a sterically hindered amine, e.g., diethanolamineas the promoter. Working capacity increases of from 20 to 60% may beobtained by use of the sterically hindered amines of the instantinvention.

While not wishing to be bound by theory, it is believed that the use ofsterically hindered amines gives the above-described improvements forthe following reasons.

When CO₂ is absorbed into an aqueous primary amine solution, thefollowing reactions occur:

    R -- NH.sub.2 + CO.sub.2 → R -- NH -- COO.sup.- + H.sup.+(1)

    r -- nh -- coo.sup.- + h.sub.2 o → r -- nh.sub.2 + hco.sub.3.sup.-(2)

    h.sup.+ + r -- nh.sub.2 → r -- nh.sub.3.sup.+       (3)

the amount of CO₂ that can be absorbed depends on the extent of reaction(2). If reaction (2) is negligible, the net result of reactions (1) and(3) will be:

    2R -- NH.sub.2 + CO.sub.2 → R -- NH -- COO.sup.- + R -- NH.sub.3.sup.+                                            (4)

i.e., the maximum amount of CO₂ that can be absorbed is 0.5 moles/moleof amine.

On the other hand, if reaction (2) is quantitative, the net result ofreactions (1), (2) and (3) will be:

    R -- NH.sub.2 + CO.sub.2 + H.sub.2 O → HCO.sub.3.sup.- + R -- NH.sub.3.sup.+                                            (5)

i.e., the maximum amount of CO₂ that can be absorbed is 1 mole/mole ofamine.

The extent of reaction (2) depends on the nature of R, particularly onits steric configuration. If R is a primary alkyl group, the carbamatewill be relatively stable and its decomposition, i.e. reaction (2), willbe incomplete. The maximum amount of CO₂ that can be absorbed will beonly slightly higher than 0.5 mols/mol of amine. On the other hand, if Ris a tertiary alkyl group, the carbamate will be very unstable and itsdecomposition, i.e. reaction (2), will be practically complete. Themaximum amount of CO₂ that can be absorbed will be close to 1 mol/mol ofamine. Thus, when the amine is sterically hindered, CO₂ absorption ismore complete than when it is unhindered.

When desorption is carried out, reactions (1), (2) and (3) go from rightto left. If R is a primary alkyl group, the decomposition of thecarbamate will be incomplete, i.e., desorption will be only partial. Onthe other hand, if R is a tertiary alkyl group, there will be no way forCO₂ to be in a stable form and desorption will be practically complete.Therefore, the amount of CO₂ absorbed or desorbed per mole of amine ishigher when the amine is sterically hindered.

If the amino group is secondary, a secondary alkyl group attached to itis enough to provide steric hindrance.

A similar explanation can be given for the increase in cyclic capacitythat is observed when a sterically hindered amine is used in conjunctionwith K₂ CO₃. In that case, the hydrogen ions formed in reaction (1)react mainly with CO₃ ⁻⁻ ions, to give bicarbonate ions, rather thanbeing trapped by the amine in reaction (3).

By increasing working capacity and rates of absorption and desorption,the use of sterically hindered amino groups leads to lower steamconsumption.

Steam requirements are the major part of the energy cost of operating anacid gas, e.g., CO₂ scrubbing unit. Substantial reduction in energy,i.e., operating costs, will be obtained by the use of the process of theinstant invention amines. Additional savings from new plant investmentreduction and debottlenecking of existing plants may also be obtained bythe use of sterically hindered amines. The removal of acid gases such asCO₂ from gas mixtures is of major industrial importance, particularlywith the systems which utilize potassium carbonate activated by thesterically hindered amines.

The larger-scale applications fall into two categories:

(a) The hydrogen industry, where hydrogen mixed with CO₂ is manufacturedfrom gas, coal or petroleum fractions; large amounts of hydrogen areused in the ammonia industry and the amount of CO₂ to be removed is ofthe order of 1.2 to 2.2 tons per ton of NH₃.

(b) The gas industry:

(1) Treatment of natural gases containing large concentrations of CO₂.

(2) upgrading of town gases manufactured from gasification of coal orreforming of petroleum fractions. A medium-size coal gasification plant,producing 250 MM SCF/D of substitute natural gas, requires removal of7-8,000 tons/day of CO₂.

The invention is illustrated further by the following examples which,however, are not to be taken as limiting in any respect. All parts andpercentages, unless expressly stated to the otherwise, are by weight.

The following is a preferred embodiment of the instant invention. TheCO₂ -containing mixture is contacted in an absorption zone with asolution comprising from about 20 to about 30 weight percent ofpotassium carbonate and at least 2 weight percent of a stericallyhindered amine at a temperature of from 35° to 150° C and a pressure offrom 100 to 500 psig for a period of from 0.1 to 60 minutes, wherebysaid solution is saturated with CO₂ to a capacity of at least 90%. Thesolution is then passed to a flash chamber, said flash chamber beingmaintained at a pressure of from 2 to 10 psig, thereby flashing off atleast some of said absorbed acid gas, and then into a regenerator towerwhereby said solution is maintained at a temperature of from about 100°to 160° C and a pressure of from about 2 to 75 psig, whereby the CO₂content of said solution is reduced to a capacity of less than 25%.Finally, said regenerated solution is passed back to said absorptionzone for reuse.

The following are specific embodiments of the instant invention.However, there is no intent to be bound thereby.

EXAMPLE 1

The experimental reaction apparatus is shown in FIG. 1. It is a vesselhaving a capacity of about 2.5 liters and a diameter of 10 cm, equippedwith a heating jacket. The stirrer shaft carries two three-bladepropellers, of which the upper one pushes the liquid downward and thelower one pushes the liquid upward. Pump P₁ removes liquid from thebottom of the reactor and feeds it back to the gas-liquid interfacethrough a stainless-steel sparger S₁. Vertical baffles further increasethe contact between liquid and gas. Thermocouple T permits the readingof the temperature of the liquid. The top of the reflux condenser C isconnected to a U-shaped, open-ended manometer M. The apparatus can beevacuated by means of pump P₂ through tap T₁. Nitrogen and CO₂ can befed to the bottom of the cell through sparger S₂, using respectivelytaps T₂ or T₃. CO₂, coming from a cylinder, goes first through the two12-1 flasks F₁ and F₂, acting as ballasts, then through a 3-1 wettest-meter WTM, then through bubbler B₁, where it is saturated withwater. Hg-bubbler B₂ insures that no air is sucked into flask F₂.

Constrictions such as narrow tubings and taps have been carefullyavoided in the CO₂ path. Tap T₃, which is the only one inserted in sucha path, has a key with large holes (8 mm).

DETAILED DESCRIPTION OF ABSORPTION-DESORPTION-REABSORPTION EXPERIMENTSA. Absorption

The following reagents are charged into the apparatus while bubblingnitrogen through tap T₂ and keeping exhaust E open and tap T₃ closed.

56 g of 2,2,5,5-tetramethyldiethylene triamine (0.35 mols)

187.5 g of K₂ CO₃ (1.35 mols)

520 ml of water

The total volume is 610 ml. The amine is incompletely soluble in theaqueous phase. The temperature of the liquid is brought to 80° C, pumpP₁ is regulated so as to suck and feed about 4 liters of liquid perminute, the stirrer is kept turning at 1200 rpm. Exhaust E and tap T₂are closed and the apparatus is evacuated by means of pump P₂ until theliquid begins to boil, which occurs when the residual pressure is about50 mm Hg. Tap T₁ is closed. At this point, tap T₃ is opened andabsorption starts. Simultaneously a timer is started. Every time thewet-test-meter WTM indicates that a liter has been absorbed, the time istaken. At the beginning the absorption is very rapid and more than 13liters of CO₂ is absorbed in the first minute. In total 34.9 liters ofCO₂ is sucked in 9 minutes. Subtracting the amount of CO₂ used to fillthe gaseous space, the amount of CO₂ absorbed is 32.9 liters, whichcorresponds to 59.5g or 1.35 moles. The total CO₂ content calculatedfrom the absorption data is 14.8% whereas that found experimentally is15.0%. Only one phase is present. The pressure is 40mm Hg.

A number of other amines were tested as K₂ CO₃ activators in absorptiontests using the equipment shown in FIG. 1 and described above. In TableI absorption rates for aqueous K₂ CO₃ activated with the stericallyhindered amines, prior art amine activators and other activated andnon-activated systems are given for various molar ratios of CO₂ absorbedto initial K₂ CO₃ (carbonation ratio). In Table II there are shown somefurther results of the absorption experiments. The CO₂ contentscalculated from the amount of gas absorbed are in good agreement withthe values determined experimentally.

                                      TABLE 1                                     __________________________________________________________________________    CO.sub.2 Absorption Rates (moles/hr) In Aqueous K.sub.2 CO.sub.3              Activated With Various Amines.sup.(a)                                                      Carbonation Ratio (CO.sub.2 absorbed/Initial K.sub.2                          CO.sub.3)                                                        Amine Activator                                                                            0.03                                                                             0.10                                                                             0.20                                                                             0.30                                                                             0.40                                                                             0.50                                                                             0.60                                                                             0.70                                                                             0.80                                                                             0.90                                                                             1.00                               __________________________________________________________________________    None         36.8                                                                             5  4  3.7                                                                              3.5                                                                              2.8                                                                              2  1  -- -- --                                 Diethanolamine                                                                             59 23 16 11 11 10 9  6.5                                                                              3  -- --                                 Catacarb.sup.(b)                                                                           59 15 12 12 10 8.5                                                                              6  2.5                                                                              -- -- --                                 GV sol'n.sup.(c)                                                                           59 13.5                                                                             13 12 10.5                                                                             9  5.2                                                                              2  --    13                                 Hexamethylenediamine                                                                       74 49 27 22 14 11 8.5                                                                              6  3  -- --                                 4-aminomethyl-piperidine                                                                   74 60 35 24.5                                                                             17.5                                                                             15.5                                                                             13 12 6.5                                                                              2  --                                 N-2-amino ethylpiperazine                                                                  74 37 31 23 15 13 12 9  6.5                                                                              -- --                                 1,2-diamino-cyclohexane                                                                    74 43 35 24.5                                                                             16 14 12 10 8  2  --                                 N-cyclohexyl-1,3-propane-                                                                  74 74 45 27 20 18 13 9.5                                                                              6.5                                                                              3  --                                 diamine                                                                       1,8-menthanediamine                                                                        59 25 23 22 20 18.5                                                                             15 12 8.5                                                                              5  7                                  bis-2-amino-propylamine                                                                    74 50 42 33 24.5                                                                             20 16.5                                                                             12.5                                                                             10.5                                                                             7  1.5                                1,7-di-sec-butyl-                                                                          74 35 31 29.5                                                                             27 22.5                                                                             20.5                                                                             15 11 6.5                                                                              --                                 diethylene triamine                                                           2,2,5,5-tetramethyl-                                                                       74 49 40 22.5                                                                             20 18.5                                                                             17.5                                                                             14 11 8  3.7                                diethylenetriamine                                                            Tetrapropylene pentamine                                                                   54 54 50 42 37 31 24.5                                                                             19.7                                                                             15.5                                                                             12 8.6                                Tetraethylene pentamine                                                                    74 49 49 37 33 28 19.7                                                                             15.5                                                                             12.5                                                                             10 5.9                                __________________________________________________________________________     .sup.(a) Experimental conditions: K.sub.2 CO.sub.3 = 187.5g; amine = 0.35     moles; water to 610 ml; temperature = 80° C. (Rates and carbonatio     ratios are not corrected for the reactor space occupied by gas).              .sup.(b) 10% solution of Catacarb 251 H was used.                             .sup.(c) GV sol'n = Giammarco-Vetrocoke solution (As.sub.2 O.sub.3 as         activator).                                                              

                  TABLE II                                                        ______________________________________                                         CO.sub.2 Absorption in Aqueous K.sub.2 CO.sub.3                              Activated With Various Amines.sup.(a)                                                              Total CO.sub.2 %                                                              After Absorption                                                        CO.sub.2 Absorbed                                                                         Calc. from                                         Amine          g           Absorption                                                                              Found                                    None           42          12.6      12.6                                     Diethanolamine 48          13.5      13.4                                     Catacarb.sup.(b)                                                                             42          12.8      12.4                                     GV Solution.sup.(c)                                                                          39.5        --        10.7                                     Hexamethylenediamine                                                                         50.5        13.7      13.6                                     4-aminomethylpiperidine                                                                      54          15.2      14.8                                     N-2-aminoethylpiperazine                                                                     51          14        14                                       1,2-diaminocyclohexane                                                                       52          14.3      13.7                                     N-cyclohexyl-1,3-                                                                            53.5        13.8      13.8                                     propanediamine                                                                1,8-p-menthanediamine                                                                        57.5        14.7      14.4                                     bis-2-aminopropylamine                                                                       58.5        14.5      14.5                                     1,7-di-sec-butyl-diethylene-                                                                 55          14.4      13.9                                     triamine                                                                      2,2,5,5-tetramethyldi-                                                                       59.5        14.8      15                                       ethylene-triamine                                                             Tetrapropylenepentamine                                                                      67          15.7      14.6                                     Tetraethylenepentamine                                                                       63          15.2      14.7                                     ______________________________________                                         .sup.(a) Experimental Conditions: K.sub.2 CO.sub.3 = 187.5g; amine = 0.35     moles; H.sub.2 O = 610 ml.                                                    .sup.(b) 10% solution of Catacarb 251 H was used.                             .sup.(c) GV sol'n = Giammarco-Vetrocoke solution (As.sub.2 O.sub.3 as         activator).                                                              

                                      TABLE III                                   __________________________________________________________________________    Influence on Different Amines on Extent of Desorption.sup.(a)                              Total CO.sub.2 %                                                              Before                                                                              After CO.sub.2 Desorbed                                                                     Decrease in                                  Amine Activator                                                                            Desorption                                                                          Desorption                                                                          g       Carbonation Ratio                            __________________________________________________________________________    None         12.6  9.3   28.5    0.50                                         Diethanolamine                                                                             13.4  9     38.5    0.65                                         Catacarb.sup.(b)                                                                           12.4  8     38      0.63                                         Hexamethylenediamine                                                                       13.6  9.1   39.6    0.66                                         4-aminomethylpiperidine                                                                    14.8  9.9   40.5    0.68                                         N-2-aminoethylpiperazine                                                                   13.5  8.9   40      0.67                                         1,2-diaminocyclohexane                                                                     13.7  9.8   33.4    0.56                                         N-cyclohexyl-1,3-propane-                                                                  13.8  8.6   46.5    0.78                                         diamine                                                                       1,8-p-menthanediamine                                                                      14.4  8.3   52.5    0.88                                         1,7-di-sec-butyl-diethylene-                                                                 13.9.sup.(c)                                                                      9.2   41      0.69                                         triamine                                                                      bis-2-aminopropylamine                                                                     14.5  9.9   41      0.69                                         2,2,5,5-tetramethyldi-                                                                     15    8.9   54      0.91                                         ethylene-triamine                                                                          15.5  9.1   57      0.96                                         Tetrapropylenepentamine                                                                    14.6  11    32.2    0.54                                         Tetraethylenepentamine                                                                     14.6  10.3  38.5    0.64                                         __________________________________________________________________________     .sup.(a) Experimental Conditions: The reaction mixture obtained after         absorption is heated to 105° C. and maintained at this temperature     for 1 hour while slowly bubbling N.sub.2 through it.                          .sup.(b) 10% solution of Catacarb 251 H was used.                             .sup.(c) The values are as reported, however, based on other experiments,     it is believed that the "13.9" was due to analytical error since the          analysis of the reabsorbed solution is 14.4% CO.sub.2, see Table IV.     

B. Desorption

Desorption of the amine(2,2,5,5-tetramethyldiethylenetriamine)-activated solution used in theabsorption experiment described above in section A is carried out asfollows:

Exhaust E is open, taps T₂ and T₃ are closed. The reaction mixture isbrought to 105° C., while stirring slowly and keeping the liquidcirculation rate at a minimum. When the temperature reaches 105° C.,nitrogen is blown from tap T₂ through sparger S₂ at a rate of 1 mole/hr.A sample of liquid is taken after 60 minutes. The total CO₂ content is8.9%, corresponding to 54 g of CO₂ desorbed. A small amount ofnon-aqueous phase is present.

Table 3 shows the results of desorptions for2,2,5,5-tetramethyldiethylenetriamine and various other amine activatedsolutions.

It can be seen from Table III that the sterically hindered amineactivated solutions can clearly better than the sterically non-hinderedamine activated solutions. None of the other amines (stericallynon-hindered) contain sterically hindered amino groups. It is to benoted that a large absorption capacity for a scrubbing solution does notnecessarily mean a large desorption. For example, by comparing Tables IIand III with one another, it can be seen that bis-2-aminopropylamine,tetrapropylenepentamine and tetraethylenepentamine lead to a very highabsorption but to a rather poor desorption. The fact that they containmany nitrogen atoms enables them to absorb a large amount of CO₂, but,as the nitrogen atoms are not of the right kind, CO₂ desorption isincomplete. Use of absorption alone as a means of screening amines, awidely used procedure, can be very misleading.

C. Reabsorption

The desorbed reaction mixture as described above (Section B) for the2,2,5,5-tetramethyldiethylene-triamine-activated solution is broughtback to 80° C., while still blowing nitrogen, the stirrer and pump P₁are regulated in the same way as for the absorption. Exhaust E and tapT₂ are closed and the apparatus is evacuated by means of pump P₂, untilthe liquid begins to boil, which occurs when the residual pressure isabout 60mm Hg. Tap T₁ is closed. Tap T₃ is opened and reabsorptionstarts. Simultaneously the timer is started. Times are taken as duringabsorption. More than 10 liters of CO₂ are consumed in the first minute.In total, 30 liters of CO₂ is sucked in 9 minutes. Subtracting theamount of CO₂ used to fill the gaseous space, the amount of CO₂reabsorbed is 28 liters, corresponding to 51g or 1.15 moles. The totalCO₂ content is 15.5%. The pressure is 32mm Hg.

Table IV shows the amounts of CO₂ reabsorbed in the presence of variousamine activators including 2,2,5,5-tetramethyldiethylenetriamine. It canbe seen that the sterically hindered amines are the best performers. InTable V reabsorption rates in the presence of various amine activatorsare shown for various molar ratios of CO₂ reabsorbed to initial K₂ CO₃.Again, the sterically hindered amine activators are the best performers.

                                      TABLE IV                                    __________________________________________________________________________    CO.sub.2 Reabsorption in The Presence of Various Amines.sup.(a)                                                       CO.sub.2                                                                      Reabsorbed                                         CO.sub.2                                                                            Increase in                                                                          Total CO.sub.2 %                                                                            g, calc                                            Reabsorbed                                                                          Carbonation                                                                          Before After  from CO.sub.2                         Amine Activator                                                                            g.    Ratio  Reabsorption                                                                         Reabsorption                                                                         Analysis                              __________________________________________________________________________    None         27    0.52   9.3    12.5   28.4                                  Diethanolamine                                                                             35.5  0.60   9      13     34                                    Catacarb.sup.(b)                                                                           38    0.64   8      12.1   35.2                                  GV sol'n.sup.(c)                                                                           35.2  0.59   6.8    10.7   --                                    Hexamethylene-                                                                             38    0.64   9.1    13.7   40.5                                  diamine                                                                       4-aminomethylpiperidine                                                                    40    0.67   9.9    15.1   43                                    N-2-aminoethylpiperazine                                                                   40    0.67   8.9    13.7   42                                    1,2-diamino-cyclohexane                                                                    37.4  0.63   9.8    13.9   35.2                                  N-cyclohexyl-1,3-propane-                                                                  45.5  0.76   9.1    12.9   46.5                                  diamine                                                                       1,8-p-menthanediamine                                                                      53.5  0.90   8.3    14     52.6                                  bis-2-aminopropylamine                                                                     40.5  0.68   9.9    14.6   42.4                                  1,7-di-sec-butyl                                                                           45    0.76   9.2    14.4   45.4                                  diethylenetriamine                                                            2,2,5,5-tetramethyl-                                                                       50.5  0.85   8.9    15.5   58.5                                  diethylenetriamine                                                            Tetraethylene pentamine                                                                    37.5  0.63   10.3   14.6   38.4                                  __________________________________________________________________________     .sup.(a) Experimental Conditions: The reaction mixture obtained after         desorption is cooled back to 80° C.                                    .sup.(b) 10% solution of Catacarb 251 H was used.                             .sup.(c) GV sol'n = Grammarco-Vetrocoke solution (As.sub.2 O.sub.3 as         activator).                                                              

                                      TABLE V                                     __________________________________________________________________________     CO.sub.2 Reabsorption Rates (moles/hr) in Aqueous K.sub.2 CO.sub.3           Activated With Various Amine Activators.sup.(a)                                            CO.sub.2 Reabsorbed/Initial K.sub.2 CO.sub.3 (Ratio)             Amine Activator                                                                            0.03                                                                             0.10  0.30                                                                             0.40                                                                             0.50                                                                             0.60                                                                             0.70                                                                             0.80                                     __________________________________________________________________________    None         29.5                                                                             3.7                                                                              3.3                                                                              3.0                                                                              1.8                                                                              .5 -- -- --                                       Diethanolamine                                                                             59 14.0                                                                             10.5                                                                             9.5                                                                              8.0                                                                              6  2.5                                                                              -- --                                       Catacarb.sup.(b)                                                                           59 13.5                                                                             12 10.5                                                                             9  6  3.2                                                                              -- --                                       GV sol'n.sup.(c)                                                                           59 14 12.5                                                                             11.8                                                                             10 7.5                                                                              2.7                                                                              -- --                                       Hexamethylenediamine                                                                       42.5                                                                             13.5                                                                             10.5                                                                             10.5                                                                             10 7.5                                                                              5.0                                                                              -- --                                       4-amino-methyl-piperidine                                                                  59 17.5                                                                             14.5                                                                             14 11 9  5.2                                                                              1.5                                                                              --                                       N-2-aminoethylpiperazine                                                                   74 21 13.5                                                                             13.3                                                                             12.5                                                                             11 7  2.2                                                                              --                                       1,2-diaminocyclohexane                                                                     59 20 15.5                                                                             13.5                                                                             11.8                                                                             9  4.9                                                                              -- --                                       N-cyclohexyl-1,3-propane-                                                                  74 32 26 21 15.5                                                                             11.5                                                                             7.5                                                                              4.9                                         diamine                                                                       1,8-p-menthanediamine                                                                      49 25.5                                                                             21 18.5                                                                             17.5                                                                             13.5                                                                             10.5                                                                             6.5                                                                              3.5                                      bis-2-aminopropylamine                                                                     59 23 18 15.5                                                                             14 11.5                                                                             6.4                                                                              4.4                                                                              --                                       1,7-bis-sec-butyl-                                                                         59 29.5                                                                             24.6                                                                             21 18.5                                                                             16 11.3                                                                             7.6                                                                              1                                        diethylenetriamine                                                            2,2,5,5-tetramethyldi-                                                                     74 26 20 17.5                                                                             15 13.6                                                                             8  7.8                                         ethylenetriamine                     5                                        Tetraethylenepentamine                                                                     59 24 17.5                                                                             14 11 8  4  -- --                                       __________________________________________________________________________     .sup.(a) Experimental conditions: The reaction mixture obtained after         desorption is cooled back to 80° C. (Rates and carbonation ratios      are not corrected for the reactor space occupied by gas.)                     .sup.(b) 10% solution of Catacarb 251 H was used.                             .sup.(c) GV sol'n = Giammarco-Vetrocoke solution (As.sub.2 O.sub.3 as         activator).                                                              

In Table VI, the amounts of CO₂ desorbed and reabsorbed in the presenceof various amines are compared. It is clear that the amount of CO₂ thatcan be desorbed or reabsorbed is higher in the case of stericallyhindered amines than in the case of Catacarb (a commercially availableCO₂ scrubbing solution), diethanolamine or tetraethylene pentamine.

In Table VII, the results of similar experiments are reported, in whichthe molar amount of amine was twice that used in the experiments ofTable VI. Again, the amounts of CO₂ desorbed or reabsorbed are higher inthe case of the sterically hindered amines. Comparison of Table VI withTable VII shows that, in the case of the sterically unhindered amines,i.e. diethanolamine, 1,6-hexanediamine and tetraethylenepentamine,essentially the same amounts of CO₂ are desorbed and reabsorbed,regardless of amine concentration or number of amino groups. On theother hand, doubling the concentration ofN-cyclohexyl-1,3-propanediamine leads to an increase in the amount ofCO₂ desorbed or reabsorbed.

                  TABLE VI                                                        ______________________________________                                        Comparison of CO.sub.2 Desorption-Reabsorption Experi-                        ments in Aqueous K.sub.2 CO.sub.3 Activated                                   With Various Amines.sup.(a)                                                                     g. CO.sub.2                                                                             g. CO.sub.2                                       Amine Activator   desorbed  reabsorbed                                        ______________________________________                                        None              28.5      27                                                Diethanolamine    38.5      35.5                                              Catacarb (10% sol'n)                                                                            38        38                                                Tetraethylene Pentamine                                                                         38.5      37.5                                              N-cyclohexyl-1,3-propane-                                                                       46.5      45.5                                              diamine                                                                       1,8-p-menthanediamine                                                                           52.5      53.5                                              1,7-bis-sec. butyl-                                                                             41        45                                                diethylenetriamine                                                            2,2,5,5-tetramethyl-                                                                            55.5      50.5                                              diethylenetriamine                                                            ______________________________________                                         .sup.(a) Experimental Conditions: K.sub.2 CO.sub.3 = 187.5G (1.35 mols);      Amine = 0.35 mols; H.sub.2 O to 610 ml.                                  

                  TABLE VII                                                       ______________________________________                                        Comparison of CO.sub.2 Desorption-Reabsorption Experi-                        ments in Aqueous K.sub.2 CO.sub.3 Activated With                              Various Amines.sup.(a)                                                                          g. CO.sub.2                                                                             g. CO.sub.2                                       Amine Activator   desorbed  reabsorbed                                        ______________________________________                                        Diethanolamine    37.6      37                                                Hexamethylenediamine                                                                            33.5      35                                                N-cyclohexyl-1,3-propane-                                                                       54        53                                                diamine                                                                       3-amino-3-methyl-1-butanol                                                                      46        45                                                ______________________________________                                         .sup.(a) Experimental Conditions: K.sub.2 CO.sub.3 = 187.5g (1.35 mols);      Amine = 0.70 mols; H.sub.2 O to 610 ml.                                  

The reabsorption test described above is carried out using Catacard andN-cyclohexyl-beta-alanine, respectively as amine activators for thepotassium carbonate solution. The results of this test are shown inTable VIII.

                  TABLE VIII                                                      ______________________________________                                        CO.sub.2 Reabsorptions                                                        The initial solutions contained 30 wt. % K.sub.2 CO.sub.3                     Initial volume: 610 ml.; T = 80° C.; P = 1 atm.                        Amine Activator, wt.                                                                            g. CO.sub.2 reabsorbed                                      ______________________________________                                        Catacarb, 75g     36                                                          N-cyclohexyl-beta-                                                                              46                                                          alanine, 120g                                                                 ______________________________________                                    

EXAMPLE 2

Some vapor-liquid equilibrium measurements were carried out to confirmthat sterically hindered amines lead to a broadening of cyclic capacityowing to a shift in the equilibrium position. In FIGS. 2 and 3 theequilibrium pressure as a function of carbonate conversion at 250° F. isshown for diethanolamine and for 1,8-p-menthanediamine andN-cyclohexyl-1,3-propanediamine-promoted potassium carbonate solutionsat equal total nitrogen content. It is clear that at low P_(CO).sbsb.2values, the carbonation ratios are very close to each other in the caseof 1,8-p-menthanediamine, whereas at high P_(CO).sbsb.2 values thecarbonation ratio is considerably higher for 1,8-p-menthanediamine andthe N-cyclohexyl-1,3-propanediamine-promoted solutions. In theP_(CO).sbsb.2 interval from 0.05 psi to 50 psi defining the typicalpressure range, the difference in the carbonation ratios correspondingto high and low P_(CO).sbsb.2 (working capacity) is about 50% larger inthe case of the 1,8-p-menthanediamine promoted solution. Theexperimental measurement of these data is described below.

The reaction apparatus is a 1-liter autoclave equipped with inlet andoutlet tube for gases. The following reagents are charged:

41g of 1,8-p-menthanediamine

125g of potassium carbonate

334g of water

The autoclave is brought to 250° F. and a gas mixture containing 20% ofCO₂ and 80% of He is slowly blown through the liquid at a pressure of200 psi. This is continued until the outgoing gas has the samecomposition as the entering gas, i.e., 20% CO₂. At this pointequilibrium between liquid and gas has been reached. A liquid sample istaken and analyzed for total CO₂ and for K. The result is 16.4% CO₂ and13.1% K. The carbonation ratio, i.e., the molar ratio of CO₂ absorbed toinitial K₂ CO₃ is 1.17.

The experiment is repeated, this time using a CO₂ -He mixture containing0.2% CO₂ and operating at 50 psi. After reaching equilibrium, analysisof the liquid phase gives a total CO₂ content of 9.7% and a K content of13.8%, from which a carbonation ratio of 0.22 is calculated. Moreexperiments are carried out under both sets of conditions. The variationof carbonation ratio between the high-pressure, high-CO₂ experiments andthe low-pressure, low-CO₂ experiments is about 0.95.

If the above experiments are repeated after replacing1,8-p-menthanediamine with a double molar amount of diethanolamine, soas to have the same nitrogen concentration, the variation in carbonationratio is only about 0.66. The working capacity advantage of1,8-p-menthanediamine is quite evident.

In Table IX, the results of the above experiments and of others carriedout under the same conditions are compared. The initial concentration ofK₂ CO₃ was 25% by weight and the total nitrogen content was 0.47g atoms.It is clear that the two non-sterically hindered amines, namelydiethanolamine and 1,6-hexanediamine, have lower working capacity thanthe sterically hindered 1,8-p-menthanediamine,N-cyclohexyl-1,3-propanediamine and 2-amino-2-methyl-1-propanol.

In Table X, results of other equilibrium experiments are given where theK₂ CO₃ concentration was again 25% and the total nitrogen content was0.94g atoms, i.e. double the concentration used for the experiments ofTable IX. Again, the sterically hindered N-cyclohexyl-1,3-propanediaminegives a 50% higher working capacity than the sterically unhinderedhexamethylenediamine.

                                      TABLE IX                                    __________________________________________________________________________    Equilibrium Experiments in the Presence of Various Amines.sup.(a)                            0.2% CO.sub.2 in gas                                                                   20% CO.sub.2 in gas                                   Amine Activator                                                                              50 psi   250 psi Difference                                    __________________________________________________________________________    Diethanolamine 0.19     0.85    0.66                                          1,6-hexamethylenediamine                                                                     0.25     0.96    0.71                                          1,8-p-menthanediamine                                                                        0.22     1.17    0.95                                          N-cyclohexyl-1,3-propane-                                                                    0.26     1.12    0.86                                          diamine                                                                       2-amino-2-methyl-1-propanol                                                                  0.15     1.08    0.93                                          2,2,5,5-tetramethyldiethylene-                                                               0.12     0.97    0.85                                          triamine (TMDET)                                                              __________________________________________________________________________     .sup.(a) Experimental Conditions: K.sub.2 CO.sub.3 = 125g; nitrogen           content of activator = 0.47g atoms; total weight of solution = 500g;          temperature = 250° F.                                             

                  TABLE X                                                         ______________________________________                                        Vapor-Liquid Equilibrium Experiments.sup.(a)                                             Carbonation Ratios                                                              0.2% CO.sub.2                                                                          20% CO.sub.2                                                                              Difference                                  Amine        in gas   in gas      (working                                    Activators   50 psig  250 psig    capacity)                                   ______________________________________                                        1,6-hexamethyl-                                                                            0.44       1.17      0.73                                        enediamine                                                                    N-cyclohexyl-                                                                              0.26       1.37      1.11                                        1,3-propane-                                                                  diamine                                                                       ______________________________________                                         .sup.(a) Experimental conditions: K.sub.2 CO.sub.3 = 125g; nitrogen           content of activator = 0.94 g. atoms; total weight of solution = 500g;        temperature = 250° F.                                             

Comparison of Table IX and Table X shows that an increase inconcentration of a sterically unhindered amine does not lead to avariation of working capacity, whereas it does in the case of asterically hindered amine. As a consequence, when comparing a stericallyhindered and a sterically unhindered amine with one another, it is notnecessary to have the same nitrogen concentration, at least withincertain limits. In the case of 2,2,5,5-tetramethyldiethylenetriamine ata total nitrogen concentration of 0.70g atoms, used in combination with25 wt. % K₂ CO₃, the working capacity is 0.85.

Further vapor-liquid equilibrium experiments with sterically hinderedamines and sterically unhindered amine activators for potassiumcarbonate scrubbing solutions are performed as described above. Theresults of these experiments are summarized in Table XI and Table XII.The data in these tables further illustrate that the use of thesterically hindered amines as potassium carbonate activatorssignificantly improves the working capacity of the scrubbing solution.

                  TABLE XI                                                        ______________________________________                                        Vapor-Liquid Equilibrium Experiments.sup.(a)                                             Carbonation Ratios                                                              0.2% CO.sub.2                                                                            20% CO.sub.2                                                                            Difference                                  Amine        in gas     in gas    (working                                    Activator.sup.(b)                                                                          50 psi     250 psi   capacity)                                   ______________________________________                                        Diethanolamine                                                                             0.18       0.92      0.74                                        2-amino-2-   0.13       1.07      0.94                                        methyl-1-                                                                     propanol                                                                      ______________________________________                                         .sup.(a) Experimental Conditions: K.sub.2 CO.sub.3 = 150g; amount of amin     activator = 0.47 mols; total weight = 500g; temperature = 250° F. 

                  TABLE XII                                                       ______________________________________                                        Vapor-Liquid Equilibrium Experiments.sup.(a)                                             Carbonation Ratios                                                              0.2% CO.sub.2                                                                            20% CO.sub.2                                                                            Difference                                  Amine        in gas     in gas    (working                                    Activator.sup.(b)                                                                          50 psi     200 psi   capacity)                                   ______________________________________                                        1,6-hexamethyl-                                                                            0.25       0.96      0.71                                        diamine                                                                       N.sub.1 -(1,1-dimeth-                                                                      0.13       1.04      0.91                                        yl-2-hydroxy-                                                                 ethyl-2-methyl-                                                               1,2-propane-                                                                  diamine                                                                       ______________________________________                                         .sup.(a) Experimental Conditions: K.sub.2 CO.sub.3 = 125g; amount of amin     activator = 0.235 mols; total weight = 500g; temperature = 250° F.

GENERAL

It can be seen from the above experimental data that the use ofsterically hindered amines broadens the working capacity of aqueous K₂CO₃ scrubbing solutions and increases the rates of CO₂ absorption andreabsorption. Sterically hindered amine activated K₂ CO₃ solutionsshould lead to lower heats of CO₂ desorption than their stericallyunhindered amine counterparts and this should lead to appreciable steamsavings.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification, and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice in the artto which the invention pertains and as may be applied to the essentialfeatures hereinbefore set forth, and as fall within the scope of theinvention.

What is claimed is:
 1. In a process for removing CO₂ from gaseous feedscontaining CO₂, which comprises:(1) contacting said gaseous feeds withan aqueous scrubbing solution comprising: (a) an alkaline materialcomprising a basic alkali metal salt or alkali metal hydroxide, (b) anactivator for said basic alkali metal salt or alkali metal hydroxidecomprising at least one amine activator for said alkaline material, and(c) water, and (2) regenerating said aqueous scrubbing solution atconditions whereby CO₂ is desorbed from said aqueous scrubbing solution,the improvement which comprises providing at least one stericallyhindered amine as at least one of the amine activators, and operatingsaid process at conditions whereby the difference between the moles ofcarbon dioxide absorbed at the end of step (1) (absorption step) and themoles of carbon dioxide absorbed at the end of step (2) (desorptionstep) would be greater at the thermodynamic equilibrium (as determinedfrom the vapor-liquid equilibrium isotherm of the reaction mixture) thanin an aqueous scrubbing process wherein diethanolamine is the only amineactivator utilized under similar conditions of gaseous feed composition,scrubbing solution composition, temperatures and pressures.
 2. Theprocess of claim 1 wherein the alkaline material is selected from thegroup consisting of potassium or sodium borate, carbonate, hydroxide,phosphate and bicarbonate.
 3. The process of claim 2 wherein thealkaline material is potassium carbonate.
 4. The process of claim 1wherein the alkaline material is present in said aqueous scrubbingsolution in an amount ranging from about 10 to about 40 weight percent,based on the total weight of the scrubbing solution.
 5. The process ofclaim 1 wherein the sterically hindered amine activator is defined as acompound containing at least one secondary amino group attached toeither a secondary or tertiary carbon atom or a compound containing aprimary amino group attached to a tertiary carbon atom.
 6. The processof claim 5 wherein the sterically hindered amine activator is selectedfrom the group consisting of aminoethers, aminoalcohols, di- andtriamines.
 7. The process of claim 5 wherein the sterically hinderedamine activator is selected from the group consisting of:N₁ -tert.butyl-1,4-pentanediamine; N₂ -isopropyl-4-methyl-2,4-pentanediamine; N₁-isopropyl-2-methyl-1,2-butanediamine;2-ethylamino-2-methyl-4-aminopentane; N-tert. pentyl-1,4-butanediamine;N-tert. butyl-1,5-pentanediamine; N₂-isopropyl-2-methyl-1,2-propanediamine; N-sec. butyl-1,3-propanediamine;N₁ -dimethyl-1,2-diamino-2-methylbutane; N-t-butyl-ethylenediamine;N-t-butyl-1,3-propanediamine; 2-methylamino-2-methyl-4-amino pentane; N₁-t-butyl-2-methyl-1,2-propanediamine; N₁-butyl-2-methyl-1,2-propanediamine; N-sec.butyl-2-methyl-1,3-propanediamine; N₁-propyl-2-methyl-1,2-propanediamine; N₁ -sec.butyl-2-methyl-1,2-propanediamine; N-t-butyl-1,4-butanediamine; N₂-ethyl-1,2-hexanediamine; 1-methyl-1-phenyl ethylenediamine;2-benzyl-1,2-propanediamine; 1-phenyl-1(2-amino-ethylamino)-propane; N₁-methyl-2-phenyl-1,2-butanediamine; N₁ -cyclohexyl-1,2-propanediamine;1-amino-1-(2-amino-isopropyl)-cyclohexane;1-methylamino-1-aminomethyl-cyclopentane;1-amino-1-aminomethylcycloheptane; N-isopropyl-1,2-diaminocyclohexane;N₂ -cyclohexyl-1,2-butanediamine; N₂ -cyclohexyl-1,2-propanediamine;N-cycloheptyl-ethylenediamine; N₁-cyclohexyl-2-methyl-1,2-propanediamine;1-(2-aminoisopropyl)-2-amino-3-methylcyclopentane;N-isopropyl-1,4-diaminocyclohexane; N₁ -cyclohexyl-N₂-methyl-ethylenediamine; N-cyclohexyl-ethylenediamine; N₁ -cyclohexyl-N₂-ethyl-ethylenediamine; N₁ -cyclohexyl-N₂ -methyl-1,2-propanediamine;N-cyclohexyl-1,3-propanediamine; 1,8-p-menthanediamine;1-amino-1-aminomethylcyclohexane; 1,3-diamino-1-methylcyclohexane; N₂-cyclohexyl-2-methyl-1,2-propanediamine; 2,4-diamino-2-methylpentane;3,5-diamino-3-methylheptane; N₁ -tert. butyl-N₂-isopropyl-1,3-propanediamine; N₁ -tert. butyl-N₂ -sec.butyl-ethylenediamine; N₁ -tert. butyl-N₂ -isopropyl-1,3-propanediamine;N₁ -tert. butyl-N₂ -butyl-ethylenediamine; N₁ -tert. butyl-N₂-isobutyl-ethylenediamine; N₁,n₂ -diisopropyl-1,2-propanediamine; N₁-tert. butyl-N₂ -isopropyl-ethylenediamine; N₁ -sec. butyl-N₂-isopropyl-ethylenediamine; N₁ -tert. pentyl-N₂-isopropyl-ethylenediamine; N₁,n₃ -diethyl-1,3-butanediamine; N₁ -tert.butyl-N₂ -methyl-ethylenediamine; N₁ -(2-pentyl)-N₂-methyl-ethylenediamine; N₁ -tert. butyl-N₂ -methyl-1,4-butanediamine;N₁ -tert. butyl-N₂ -ethyl-1,3-propanediamine; N₁ -tert. butyl-N₂,N₂-diethyl ethylenediamine N₁ -cyclohexyl-dipropylene triamine; N₁,n₃,2-pentamethyl-1,2,3-triaminopropane; N₁ -isopropyl-N₂-(3-aminopropyl)-2-methyl-1,2-propanediamine; 2.2-dimethyl-diethylenetriamine;N₁ -tert. butyl-1,2,3-triaminopropane;2,2,5,5-tetramethyldiethylenetriamine;1-amino-1-aminomethyl-2-hydroxymethylcyclohexane;N-hydroxyethyl-1,2-diaminocyclohexane;N-cyclohexyl-1,3-diamino-2-propanol;N-(2-hydroxycyclohexyl)-1,3-propanediamine;N-isopropanol-1,2-diaminocyclohexane;N-(2-hydroxybutyl)-1,4-diaminocyclohexane; N₁(1-hydroxy-2-butyl)-2-methyl-1,2-propanediamine;N(1-hydroxy-2-methyl-2-butyl)-1,3-propanediamine; N₁(1,1-dimethyl-2-hydroxyethyl)-2-methyl-1,2-propanediamine; N₃-isobutyl-2-methyl-2,3-diamino-1-propanol;N(3-hydroxy-2-butyl)-2,3-diaminobutane; N₁-hydroxyethyl-2-methyl-1,2-propanediamine; 2,N₃,N₃-trimethyl-2,3-diamino-1-propanol; N₁,2-dimethyl-N₁-hydroxyethyl-1,2-propanediamine;N(1,1-dimethyl-2-hydroxyethyl)-1,3-propanediamine; N-tert.butyl-1,3-diamino-2-propanol; 3-amino-3-methyl-2-pentanol;1-hydroxymethyl-cyclopentylamine; 2,3-dimethyl-3-amino-1-butanol;2-amino-2-ethyl-1-butanol; 1-methyl-2-hydroxycyclopentylamine;2-amino-2-methyl-3-pentanol; 2,4-dimethyl-2-amino cyclohexanol;1-hydroxyethyl cyclohexylamine; 1-hydroxymethyl-3-methylcyclohexylamine; 2-hydroxymethyl-1-methyl-cyclohexylamine;2-amino-2-methyl-1-propanol; 2-amino-2-methyl-1-butanol;3-amino-3-methyl-1-butanol; 3-amino-3-methyl-2-butanol;2-amino-2,3-dimethyl-3-butanol; 2-amino-2,3-dimethyl-1-butanol;1-hydroxymethylcyclohexylamine; 2(2-amino-2-methylpropoxy)-ethanol;2-piperidine methanol; 2-piperidine ethanol;2-(1-hydroxyethyl)-piperidine; 5-hydroxy-2-methyl piperidine;2-methyl-3-hydroxy piperidine; 2,6-dimethyl-3-hydroxy piperidine;2,5-dimethyl-4-hydroxy piperidine; 2-tert.butylamino-methyl-1,4-dimethyl-piperazine; 1-tert.butylamino-3,6-dimethyl-3,6-diaza-cycloheptane; andN-cyclohexyl-beta-alanine.
 8. The process of claim 5 wherein thesterically hindered amine activator is selected from the groupconsisting of: N-cyclohexyl-1,3-propane-diamine; 1,8-p-menthane-diamine;1,7-bis-sec-butyl-diethylenetriamine; 2,2,5,5-tetramethyldiethylenetriamine; 3-amino-3-methyl-1-butanol;2-amino-2-methyl-1-propanol; N₁ -isopropyl-2-methyl-1,2-propane diamine;N₁ -(1,1-dimethyl-2-hydroxyethyl)-2-methyl-1,2-propanediamine; and N₁-isopropyl-N₂ -(3-aminopropyl)-2-methyl-1,2-propanediamine.
 9. Theprocess of claim 5 wherein said sterically hindered amine isN-cyclohexyl-1,3-propanediamine.
 10. The process of claim 1 wherein saidscrubbing solution additionally includes an additive selected from theantifoaming agents, antioxidants, corrosion inhibitors and mixturesthereof.
 11. The process of claim 1 wherein the scrubbing solution whichhas been desorbed in reused to absorb a feed containing CO₂.
 12. Aprocess for removing CO₂ from gaseous feeds containing CO₂ whichcomprises contacting said feed with an aqueous solution comprising (a)potassium carbonate in an amount ranging from about 10 to about 40weight percent, (b) at least about 2 weight percent of an amineactivator comprising a sterically hindered amine selected from the groupconsisting of: N-cyclohexyl-1,3-propanediamine; 1,8-menthanediamine;1,7-bis-sec-butyl-diethylenetriamine; 2,2,5,5-tetramethyldiethylenetriamine; 3-amino-3-methyl-1-butanol;2-amino-2-methyl-1-propanol, N₁ -isopropyl-2-methyl-1,2-propanediamine;N₁ (1,1-dimethyl-2-hydroxyethyl)-2-methyl-1,2-propanediamine; and N₁-isopropyl-N₂ -(3-aminopropyl)-2-methyl-1,2-propanediamine.
 13. Theprocess of claim 12 wherein said contacting takes place at a temperatureranging from about 35 to about 150° C. and at a pressure ranging fromabout 100 to 500 psig for a period of from 0.1 to about 60 minutes. 14.The process of claim 12 wherein said scrubbing solution is regeneratedat a temperature ranging from about 100° to 160° C. and at a pressureranging from about 2 to about 75 psia. is reduced to a capacity of lessthan 25%.
 15. A process for removing CO₂ from gaseous feeds containingCO₂ which comprises contacting said feed with an aqueous solutioncomprising: (a) potassium carbonate in an amount ranging from about 10to about 40 weight percent, (b) at least 2 weight percent of an amineactivator comprising N-cyclohexyl-1,3-propanediamine and (c) water. 16.The process of claim 15 wherein said aqueous scrubbing solutionadditionally includes an additive selected from the group consisting ofantifoaming agents, antioxidants, corrosion inhibitors and mixturesthereof.
 17. The process of claim 12 wherein said aqueous scrubbingsolution includes a sterically nonhindered amine.
 18. In a process forremoving carbon dioxide from gaseous feeds containing carbon dioxidewhich comprises, in sequential steps:(1) contacting said feed with anaqueous scrubbing solution comprising: (a) an alkaline materialcomprising a basic alkali metal salt or alkali metal hydroxide in anamount ranging from about 10 to about 40 weight percent, (b) at least 2weight percent of an amine activator, and (c) water, at conditionswhereby said carbon dioxide is absorbed in said scrubbing solution, and(2) regenerating said scrubbing solution at conditions whereby carbondioxide is desorbed from said scrubbing solution, the improvement whichcomprises providing a sterically hindered amine as an amine activator,and operating said process at conditions whereby the difference betweenthe moles of carbon dioxide absorbed at the end of step (1)(absorptionstep) and the moles of carbon dioxide absorbed at the end of step(2)(desorption step) would be greater at the thermodynamic equilibrium(as determined from the vapor-liquid equilibrium isotherm of thereaction mixture) than in an aqueous scrubbing process whereindiethanolamine or 1,6-hexanediamine is the only amine activator utilizedunder similar conditions of gaseous feed composition, scrubbing solutioncomposition, temperatures and pressures.
 19. The process of claim 18wherein said alkaline material is selected from the group consisting ofpotassium or sodium borate, carbonate, hydroxide, phosphate andbicarbonate.
 20. The process of claim 19 wherein said alkaline materialis potassium carbonate.
 21. The process of claim 18 wherein thesterically hindered amine activator is defined as a compound containingat least one secondary amino group attached to either a secondary ortertiary carbon atom or a compound containing a primary amino groupattached to a tertiary carbon atom.
 22. The process of claim 21 whereinthe sterically hindered amine activator is selected from the groupconsisting of aminoethers, aminoalcohols, di- and triamines.
 23. Theprocess of claim 22 wherein the sterically hindered amine activator isselected from the group consisting of: N-cyclohexyl-1,3-propanediamine;1,8-p-menthanediamine; 1,7-bis-sec-butyl-diethylenetriamine;2,2,5,5-tetramethyl diethylenetriamine; 3-amino-3-methyl-1-butanol;2-amino-2-methyl-1-propanol; N₁ -isopropyl-2-methyl-1,2-propanediamine;N₁ -(1,1-dimethyl-2-hydroxyethyl)-2-methyl-1,2-propanediamine; and N₁-isopropyl-N₂ -(3-aminopropyl)-2-methyl-1,2-propanediamine.
 24. Theprocess of claim 22 wherein the sterically hindered amine activator isN-cyclohexyl-1,3-propanediamine.
 25. The process of claim 18 wherein theprocess is operated at conditions whereby the difference between themoles of carbon dioxide absorbed at the end of step (1)(absorption step)and the moles of carbon dioxide absorbed at the end of step(2)(desorption step) would be at least 15% greater at the thermodynamicequilibrium (as determined from the vapor liquid equilibrium isotherm ofthe scrubbing solution) than in an aqueous scrubbing process whereindiethanolamine or 1,6-hexanediamine is the only amine activator utilizedunder similar conditions of gaseous feed composition, scrubbing solutioncomposition, temperatures and pressures.
 26. The process of claim 18wherein said scrubbing solution additionally includes an additiveselected from the group consisting of antifoaming agents, antioxidants,corrosion inhibitors and mixtures thereof.
 27. The process of claim 18wherein the scrubbing solution from step (2) which has been desorbed inreused to absorb a feed containing carbon dioxide.
 28. The process ofclaim 18 wherein said contacting in step (1)(absorption step) takesplace at a temperature ranging from about 25° to about 200° C. and at apressure ranging from about 5 to about 2000 psig for a period rangingfrom 0.1 to 60 minutes.
 29. The process of claim 18 wherein thescrubbing solution is regenerated at a temperature ranging from 25° to200° C. and at a pressure ranging from about 16 to about 100 psia. 30.The process of claim 1 wherein said gaseous feed contains CO₂ incombination with H₂ S.
 31. The process of claim 6 wherein saidsterically hindered amine is selected from the group consisting ofmonoprimary aliphatic diamines; arylaliphatic diamines in which theamino groups are separated by up to 5 or more than 6 carbon atoms;cycloaliphatic diamines; biprimary aliphatic diamines in which thenitrogen atoms are separated by up to 5 or more than 6 carbon atoms;disecondary aliphatic diamines; secondary-tertiary diamines;cycloaliphatic triamines; aliphatic triamines containing at most oneprimary amino group; biprimary or triprimary aliphatic triamines inwhich any two vicinal groups are separated by up to 5 or more than 6carbon atoms; cycloaliphatic diaminoalcohols; diaminoalcohols-containingat most one primary amino group; sterically hindered primarymonoaminoalcohols; sterically hindered amino-hydroxyalkylethers;2-substituted piperidines; piperazine derivatives; azacyclo alkanes; andaminoacids.
 32. The process of claim 6 wherein said sterically hinderedamine is a cycloaliphatic diamine.
 33. In a process for removing CO₂from gaseous feeds containing CO₂ which comprises, in sequentialsteps:(1) contacting said feed with an aqueous scrubbing solutioncomprising (a) from about 20 to about 35 weight percent of potassiumcarbonate, (b) from about 2 to about 20 weight percent of an amineactivator, and (c) water, at conditions whereby said CO₂ is absorbed insaid scrubbing solution; (2) regenerating said scrubbing solution atconditions whereby carbon dioxide is desorbed from said scrubbingsolution, the improvement which comprises providing an asymmetricalsterically hindered cycloaliphatic diamine as an amine activator whereinthe sterically hindered cycloaliphatic diamine is defined as a compoundhaving a boiling point of at least 80° C. and containing at least onesecondary amino group attached to a secondary carbon atom.
 34. Theprocess of claim 33 wherein said scrubbing solution additionallyincludes an additive selected from the group consisting of antifoamingagents, antioxidants, corrosion inhibitors and mixtures thereof.
 35. Theprocess of claim 33 wherein the scrubbing solution from step (2) whichhas been desorbed is reused to absorb a feed containing carbon dioxide.36. The process of claim 33 wherein said contacting in step (1)(absorption step) takes place at a temperature ranging from about 25° toabout 200° C. and at a pressure ranging from about 5 to about 2000 psigfor a period ranging from 0.1 to 60 minutes.
 37. The process of claim 33wherein the scrubbing solution is regenerated at a temperature rangingfrom 25 to 200° C. and at a pressure ranging from about 16 to about 100psia.
 38. The process of claim 33 wherein the gaseous feed contains CO₂and H₂ S.